THE EFFECT OF pH IN INTENSIVE MICROALGAL CULTURES. I. BIOMASS REGULA TION

Two freshwater and two marine alg;tl species were grown in intensive contiimous cultures at a fixed dilution rate of 0.5 ·day -I, but at varying pH levels in the range 7.6 to 1C.6. Both freshwater species, Scenedesmus obliquus (Turp.) Kutz. and Chlorella vulgaris Beij., grew up to pH 10.6 although C. vulgaris was more adversely affected by alkaline pH than was Scenedesmus obliquus. Of the marine species, Phaeodactylum tricornutum (TFX-1) Bohlin was hardly affected by varying pH up to its maximum tolerable level of 10.3, whereas growth of Dunaliella tertiolecta (Dun) Butcher was adversely affected by increasing pH and ceased when the pH exceeded 9.3. These results are consistent with the general observations that many marine species cannot tolerate alkaline pH values much above 9.5. Moreover, the unique ability of Phaeodactylum tricornutum to grow at pH > 10 probably is a major factor contributing to its well documented success in large-scale outdoor cultures that are poorly buffered. It is difficult to .separate metabolic from purely chemical factors that influence the pH tolerance limits of the individual species. The lower pH limits were, however, distinctly controlled by the production of alkalinity concomitant with No3uptake, whereas the upper pH limits in the case of Scendesmus obliquus and Phaeodactylum tricornutum seemed to be regulated primarily by metabolic control. In no case was the availability of inorganic carbon an influencing factor in setting the maximum attained pH.